Abstract
Hydrocalumite is a layered double hydroxide (LDH) that is finding increased application in numerous scientific fields. Typically, this material is produced through environmentally polluting methods such as co-precipitation, sol-gel synthesis and urea-hydrolysis. Here, the hydrothermal green (environmentally friendly) synthesis of hydrocalumite (CaAl-OH) from Ca(OH)2 and Al(OH)3 in water and the parameters that influence its formation are discussed. The parameters investigated include the reaction temperature, reaction time, molar calcium-to-aluminium ratio, the morphology/crystallinity of reactants used, mixing and the water-to-solids ratio. Hydrocalumite formation was favoured in all experiments, making up between approximately 50% and 85% of the final crystalline phases obtained. Factors that were found to encourage higher hydrocalumite purity include a low water-to-solids ratio, an increase in the reaction time, sufficient mixing, the use of amorphous Al(OH)3 with a high surface area, reaction at an adequate temperature and, most surprisingly, the use of a calcium-to-aluminium ratio that stoichiometrically favours katoite formation. X-ray diffraction (XRD) and Rietveld refinement were used to determine the composition and crystal structures of the materials formed. Scanning electron microscopy (SEM) was used to determine morphological differences and Fourier-transform infrared analysis with attenuated total reflectance (FTIR-ATR) was used to identify possible carbonate contamination, inter alia. While the synthesis was conducted in an inert environment, some carbonate contamination could not be avoided. A thorough discussion on the topic of carbonate contamination in the hydrothermal synthesis of hydrocalumite was given, and the route to improved conversion as well as the possible reaction pathway were discussed.
Highlights
IntroductionWhere x is the ratio of trivalent to total cations in the layered double hydroxide (LDH) lattice, X is the interlayer anion, q its charge and n the amount of water present in the interlayer [1]
Hydrocalumite (HC) is a CaAl-LDH with the general formula [Ca1− x Alx (OH)2 ][Xx/q q− · nH2 O] (1)where x is the ratio of trivalent to total cations in the layered double hydroxide (LDH) lattice, X is the interlayer anion, q its charge and n the amount of water present in the interlayer [1]
The only externally variable parameters during the reaction are those that will be discussed in this paper and the use of other reactants, such as oxides, etc. which will not be discussed here
Summary
Where x is the ratio of trivalent to total cations in the layered double hydroxide (LDH) lattice, X is the interlayer anion, q its charge and n the amount of water present in the interlayer [1]. HC occurs corrugated-iron-like structure, the ratio of Ca:Al is limited to 2:1 (x = 0.33). As with other LDHs for study in applications, this material is typically produced using co-precipitation—the most widely used synthesis technique for LDHs [1,17]. While co-precipitation has many advantages, such as simplicity, speed and high tailorability of the materials produced, it is one of the most polluting synthesis methods available to produce LDHs, causing large amounts of salt-rich waste water
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